1. Field of the Invention
The present invention relates to a semiconductor device in which an ohmic electrode is formed on a substrate made of silicon carbide with a large bandgap, and a method for producing the same.
2. Description of the Related Art
In recent years, a semiconductor made of silicon carbide (SiC) is drawing attention as a next-generation semiconductor material due to its physical advantage of a wide bandgap and the substantially unlimited availability of its constituent elements. SiC has a crystal structure formed by a covalent bond, so that it is very stable physically and has a large bandgap. Therefore, a Schottky contact can be formed easily on a junction surface between metal and SiC, whereas it is difficult to form an ohmic contact thereon. In order to form an ohmic contact, it is required to select a material appropriately and conduct a heat treatment at a very high temperature.
Hereinafter, a method for forming an ohmic electrode using a conventional construction will be described with reference to the drawings. FIG. 6 is a cross-sectional view showing a configuration of a field-effect transistor that is one of the conventional SiC semiconductor devices. FIGS. 12A to 12D are cross-sectional views illustrating the processes of a method for producing the field-effect transistor. First, as shown in FIG. 12A, a SiC member 62 doped with an impurity in a low concentration, and a SiC member 63 doped with an impurity in a high concentration are formed on the upper surface of a SiC substrate 61 by crystal growth. Then, as shown in FIG. 12B, a part of the SIC member 63 that is the uppermost member is removed to expose the SiC member 62. Thereafter, as shown in FIG. 12C, ohmic electrodes 68 are formed on the SiC member 63, and a heat treatment is conducted at a high temperature, whereby an ohmic contact is obtained. The ohmic electrodes 68 will function as a drain electrode and a source electrode. Furthermore, as shown in FIG. 12D, a gate electrode 69 is formed on the SiC member 62 to obtain a Schottky contact.
As a result of the above-mentioned processes, a SiC field-effect transistor with a conventional construction as shown in FIG. 6 is completed. A part of the SiC member 63 may be removed after the ohmic electrodes 68 are formed. An ohmic contact generally is obtained by inserting the SiC substrate 61 into a heating coil of a high-frequency heating furnace, and conducting a heat treatment at a high temperature of about 1000° C. to 1600° C. This method is disclosed by, for example, C. Arnodo et al., “Nickel and Molybdenum Ohmic Contacts on Silicon Carbide”, Institute of Physics Conference Series No. 142, pp. 577–580, 1996 and the like.
However, according to the above-mentioned method for forming an ohmic electrode with the conventional construction, a heat-treatment temperature is much higher than heat-resistant temperatures of conventional semiconductor materials such as Si and GaAs, and the resistance of an ohmic contact thus obtained also is high. In addition, a metal material for an ohmic electrode needs to have a melting point higher than the heat-treatment temperature, so that the selection is limited to refractory metals and the like. Furthermore, this heat-treatment temperature is close to a growth temperature of SiC crystal and an annealing temperature for activation conducted after ion implantation. This may degrade the crystal structure and cause an impurity to diffuse again. In terms of facility, the conventional method also has various problems. More specifically, the conventional method requires a special apparatus such as a high-frequency heating furnace for conducting a heat treatment at a high temperature, complicated management of a temperature and an atmospheric gas, safety management with respect to a high temperature, and the like. These problems hinder the practical use and mass-production of a SiC semiconductor device.